Traditionally, used wood, paper and agricultural byproducts, such as sawdust, woodwaste, corncobs, straw, sugar cane bagasse, newspaper and the like have been regarded essentially as waste materials, and have been disposed of through incineration or by other, similarly unproductive, means. It is well known that the lignocellulosic constituents of such materials can be hydrolyzed to produce more valuable products which in turn can be converted into additional and different valuable products; however, such operations are in limited use, due largely to the relatively low returns on investment which they have been capable of generating. The capital expenditures required to design and construct the facilities for carrying out such recovery operations tend to be significant, thus demanding that relatively high conversion rates be attainable in order to justify the expense involved.
In U.S. Application Ser. No. 2,885 filed Jan. 12, 1979 now, U.S. Pat. No. 4,201,596 issued May 6, 1980 of John A. Church et al entitled "Continuous Process For Cellulose Saccharification" and commonly assigned herewith, there is described a method and apparatus for saccharification of cellulosic products in which the cellulosic constituents of typical waste products may be converted into glucose, furfural and xylose. Such a process conveniently, rapidly and economically provides by acid cellulose hydrolysis, a hydrolyzate which may be used as the raw material for the production of more valuable products. For example, as disclosed in said U.S. Pat. No. 4,201,596 such hydrolyzates may be used as the raw fermentable substance in a process for converting sugar into ethyl alcohol.
While it has long been known that cellulosic hydrolyzate solutions may be made fermentable for the production of alcohols, prior procedures have been feasible only on a small laboratory scale and have not significantly developed beyond this stage. Among the principal conditions contributing to this state of the art has been the inordinately slow rates of reaction with extremely low yield and the lack of predictability of conditions that would permit fermentation with any given hydrolyzate. Additionally, the economic considerations inherent in the chemical conversion of sugar to alcohol have been a limiting factor. For example, it is only theoretically possible to obtain one unit of alcohol from every two units of sugar present in the raw material. Losses in sugar content of the raw material through process conditions, mechanical processing, etc., serve to even further decrease the low yields that it has been possible to obtain.
Cellulosic extractives and their decomposition products as fermentable raw materials have been particularly enigmatic to approach because of the wide variety of factors, many of which are unknown, that adversely affect and, in many cases, prevent the fermentation process. One factor that has long been recognized in the art as significantly retarding the development of a feasible fermentation process has been the presence of materials in the hydrolyzate that act as toxins or fermentation inhibitors. However, the toxins present in any given hydrolyzate may vary considerably depending on its processing history, its source, etc. Moreover, the problem is further compounded by the fact that even after the particular toxins have been identified in a given hydrolyzate, their action under any given set of conditions has been largely unpredictable and fermentation has been difficult even under special conditions. Various workers in the art have suggested that these difficulties may be dependent on any number of factors including processing temperatures, pH of the media, the presence or absence of oxygen, the concentration and type of toxin substance, the ratio of yeast cells to toxin substances, the physiological condition of the yeast cells, the wide variation in the toxicity of various substances on the metabolism of the particular yeast, the oxidation-reduction potential developed during reaction, and many other factors. Discussions of the various difficulties of fermentation and general factors influencing fermentation are found in many sources in the literature.
Cumulative discussions are given by Harris et al in "Fermentation of Douglas Fir Hydrolyzates by S. cerevisiae" and Leonard et al, "Fermentation of Wood Sugars to Ethyl Aclohol;" Industrial and Engineering Chemistry, Vol. 38, pp. 896 to 904, (1946) and Vol. 37, pp. 390 to 397, (1946), respectively.
Other workers in the art include Eklund et al, "Acid Hydrolysis of Sunflower Seed Husks for Production of Single Cell Protein," European Journal of Applied Microbiology, Vol. 2, pp. 143-152 (1976) who disclose a method of hydrolyzing sunflower seed husks and degradation of the resulting hydrolyzate to produce protein.
German Pat. No. 676,967 to Scholler (1939) describes a method for clarifying xylose worts obtained by acid hydrolysis of cellulose-containing substances for feed purposes or yeast production by precipitating calcium phosphate and calcium sulfate after heating to 65.degree. to 100.degree. together with centrifuging and conducting the wort over oxidized metal fillings or large surface area materials while the wort is at a pH of 4 to 7.5, adding malt sprouts to the thus clarified wort and stirring for several hours.
U.S. Pat. No. 2,203,360 dated June 4, 1940 to Partansky discloses a method for improving the fermentation characteristics of acid wood hydrolyzates by treating the hydrolyzate with lime to adjust the pH to between 9 and 10, aging for 1 to 2 days, reducing the pH with sulfuric acid to pH 5, purifying the solution with activated charcoal, diluting the solution to contain 40-70% by volume of hydrolyzate, inoculating the solution with yeast culture and fermenting for 2 days.
The prior art, as represented by the methods discussed above, is illustrative of the absence of a feasible commercial process for fermentation of acid hydrolyzates to alcohols due to inordinately slow reaction times and low yields and/or the lack of direction for obtaining the same. A method for readily and efficiently producing alcohol by fermentation of sugars present in wood and wood-byproducts is a particularly timely and significant development in view of current interest in alcohol as a potential energy source available from renewable raw materials.
A primary object of this invention is to provide a process for fermentation of sugars present in acid hydrolyzates derived from lignocellulosic materials.
Another object of the invention is to provide such a process in which reaction times are relatively short, in which fermentation may be effected at relatively high sugar concentrations and in which control mechanisms are established which permit predictability, reproduction of results with consistency and production of end products of high value.
Another object of this invention is to provide a process in which the acid hydrolyzate of cellulosic waste materials may be converted into ethyl alcohol.
The accomplishment of these and other objects will be apparent from the description of the invention which follows: